Extensions are different from plugins, which help the browser display specific content like playing multimedia files. Extensions are also different from search plugins, which plug additional search engines in the search bar..

If you end up writing a gem, dont forget to enter it into Mozilla's extension competition - Extend Firefox 3! This is now closed, here are the results.

Interestingly, For GreaseMonkey fans or users, there's also the possibility of tinkering with the interfaces of the Google services (Eg. Google mail) using Google's Experimental GreaseMonkey API. This allows you to literally re-organize the service as you see it. Another reason to just love Google! The best part of this arrangement is that all it uses the Javascript syntax and is written in a text file (.JS) using your favourite editor, making it a breeze to prototype and even maintain. Here's a example script to give you the general idea.

If you come up with some interesting scripts or add-ons, dont forget to post a link as comments. I am sure me and other readers will enjoy, both, looking through and using them :-)